Studies are proposed to determine a nontoxic level and source of dietary selenium which affords maximal protection against l-methyl-l-nitrosourea and 7,12-dimethylbenz (alpha) anthracene-induced mammary carcinogenesis. The inhibitory effect of selenium on the initiation and progression stages of carcinogenesis will be studied both in mammary gland organ culture and via feeding studies. The role of alterations in both the metabolism of steroid sex hormones and the synthesis of DNA and protein will be investigated as possible mechanisms by which selenium exerts its effect. Additional studies will be conducted to answer questions pertinent to the application of selenium treatment to breast cancer control and prevention.